fu'ivla x1 (plural of ordered pairs) is an exhaustive set (possibly infinite) of inputs/outputs defining a unary function which yields result x3 when given input x2; x1(x2) = x3 "f(x) = z" == "fy funtiio xy fi zy". "lo ro se zilbri be lo ka ce'u sumji ce'u li pa" is a way to extract the input/output set of the relation "f(x) = x−1", so that you can use it as funtiio's x1. Using a la-name for referring to a function is also an option, for example "la .fankcyn. cu funtiio li re li ci". See also fancu, fancuka.